Allow no grouping

If no grouping is specified, Team Mode manual allocation crashed, this
is now fixed so that all groups are shown.
Reverting change to prepare_grading_report_data

Was including feedback author, but this is incompatible with team mode,
so we will use a separate database call. It’s not the right place to do
it.
Fixed a problem where uncompleted examples would crash the page
More language fields, spelling correction

“examples assessments”
Examples comparison more obvious distinction between Reference and Submitter's assessment
Add more info for students in Closed phase
Add team mode support for CSV download